Capacitor

ABSTRACT

A capacitor includes a conductive tube, covered by insulating material and a layer of conductive material located at the central part of the tube. The geometry presents a low inherent inductance and a continuous, distributed conductance. The capacitor may be used as a bypass capacitor in an IOT amplifier to reduce stray and leakage high frequency radiation, in one amplifier being located within the inner wall of an annular input cavity.

FIELD OF THE INVENTION

[0001] This invention relates to a capacitor and more particularly, butnot exclusively, to a bypass capacitor for use in an inductive outputtube (IOT).

BACKGROUND TO THE INVENTION

[0002] IOTs are electron beam tube devices used, for example, to amplifyhigh frequency signals for TV broadcasting and require voltages in theregion of some tens of kilovolts for this operation.

SUMMARY OF THE INVENTION

[0003] According to the present invention, there is provided a capacitorcomprising an electrically conductive tube, a layer of insulatingmaterial on the outer surface of the tube and electrically conductivematerial on the insulating material.

[0004] The geometry of a capacitor in accordance with the inventionleads to a device having an inherently low inductance and also providesa continuous, distributed capacitance, making the capacitor particularlysuitable for use as a bypass capacitor in high frequency electron beamtubes such as IOT amplifiers. This gives an improvement, for example, insideband performance when used in digital television applications.

[0005] In a particularly advantageous embodiment of the invention, thetube provides support for the layer and the conductive material. As thetube acts as a substrate, neither the layer nor conductive material needbe self-supporting. Advantageously, the insulating material has agreater length in the longitudinal axial direction of the tube than theelectrically conductive material. Thus, the electrical path lengthbetween the tube and the conductive material may be made relativelylong, providing good voltage hold-off.

[0006] Preferably, the insulating material covers substantially thewhole of the outer surface of the tube. It may be applied for example bydip coating or in any other suitable way for the materials involved. Oneinsulating material of particular suitability for the capacitor isKapton (Trade Name) but other high dielectric constant insulatormaterials may be used. The Kapton may be applied as one or more layerson the outer surface of the tube or may be dip coated. The conductivematerial may be laid down on the insulating material by spraying, forexample, or in another arrangement may be a metal foil wrapped aroundit.

[0007] Preferably the tube, insulating material and conductive materialare arranged in a circularly symmetrical geometry about the longitudinalaxis of the tube to minimize inductance. In one preferred arrangement,the length of the tube is at least twice its diameter.

[0008] The tube, being hollow, may if necessary surround electricalleads or other components of a device.

[0009] Advantageously, an electrical connector is included which isconnected to the conductive material at substantially the mid-point ofthe axial extent of the conductive material. The connector may besubstantially annular, making a connection with the conductive materialaround the inner periphery of the connector, again leading to lowinductance. The connector may be an annular member arranged extensive ina plane normal to the longitudinal axis. In one embodiment, theconnector includes apertures therein by which, for example, cooling airmay be directed to other components of a device to which it isconnected. The connector may include a mesh. The mesh may form only apart of the connector to provide a fluid path or it may make up a moresubstantial amount of the connector. For example, the connector mayconsist of a mesh arranged about a frame. The apertured or meshconfiguration of the connector also give a reduction in weight which maybe desirable in some uses.

[0010] In one embodiment the connector is electrically connected to theconductive material via a plurality of spring fingers. Similarly, theconnector may also have a plurality of spring fingers around its outerperiphery by which it makes electrical connection with a surroundingconductive member. The fingers provide good electrical connection butalso permit the position of the connector to be adjusted readily. Theconnector may be supported by a separate electrically insulating supporton which it is mounted.

[0011] A capacitor in accordance with the invention is particularlysuitable for use where it is required to hold off high voltages, of theorder of several tens of kilovolts. In one typical arrangement, the tubeof the capacitor is maintained at 35 kV and the conductive material atground potential during operation of a device to which it is connected.

[0012] According to a feature of the invention, an electron beam tubecomprises an electron gun having a cathode and a capacitor in accordancewith the invention connected as a bypass capacitor to short circuitstray and leakage high frequency power. The tube of the capacitor ispreferably connected to cathode potential. Where the electron beam tubeis combined with an annular input resonant cavity as in an IOTamplifier, the capacitor may conveniently be located in the volumesurrounded by the inner wall of the cavity and substantially parallelthereto. The conductive material of the capacitor may be electricallyconnected to the inner wall.

BRIEF DESCRIPTION OF DRAWINGS

[0013] One way in which the invention may be performed is now describedby way of example with reference to the accompanying drawings in which:

[0014]FIG. 1 schematically shows a capacitor in accordance with theinvention and included in an IOT; and

[0015]FIG. 2 schematically shows the IOT of which that shown in FIG. 1is a part.

DESCRIPTION OF A PREFERRED EMBODIMENT

[0016] With reference to FIGS. 1 and 2, a capacitor 1 included in aninductive output tube amplifier includes a cylindrical hollow aluminiumtube 2 having a length which is almost three times its diameter. Theouter surface of the aluminium tube 2 is covered by three layers ofKapton 3 at the mid-point in the axial direction of the tube 2, and aband of metal foil 4 is wrapped around the outside of the Kapton 3. Thetube 2 thus acts as a substrate for the Kapton 3 and metal foil 4.During use, the tube 2 acts as one plate of the capacitor and the foil 4as the other plate. The Kapton may alternatively be applied to the tube2 by dip coating instead of using Kapton sheets.

[0017] The aluminium tube 2 is supported at one end by part of astructure 5 which in turn is connected to the cathode 6 of the IOT, thesupport structure 5 being maintained at cathode potential duringoperation of the RF amplifier.

[0018] An annular metallic plate 7 surrounds the tube 2 of thecapacitor, being arranged normal to its longitudinal axis. The innerperiphery of the annular plate 7 includes a plurality of conductivespring fingers 8 located around its inner circumference. These provide agood electrical connection between the metal foil 4 and the metallicannular plate 7. The plate 7 is supported on an insulating cylinder 9.

[0019] The IOT includes an annular input cavity 10 having an inner wall11 and an outer wall 12. The connector 7 is electrically connected viaanother plurality of spring fingers 13 arranged equidistantly around itsperiphery to the inner wall 11 of the input cavity 10. Thus, the metalfoil 4 is electrically connected to the inner wall 11 of the inputcavity 10 which during use of the amplifier is maintained at groundpotential. Typically, cathode potential is 35 kV and thus the insulatinglayer 3 must reliably hold off a voltage of 35 kV. The end of aluminiumtube 2 remote from the cathode support 5 is capped by a curvedanti-corona member 14. Alternatively, this could be provided by suitablyshaping the end of the tube 2 itself. The spring fingers permitelectrical contact to be maintained whilst the plate 7 is moved in anaxial direction. In other variants the plate 7 may be connected to thefoil 4 and/or inner wall 11 by a fixed connection if movement is notrequired.

[0020] The plate 7 is apertured to allow cooling air to be flowed overthe inner wall 11. Part of the plate also includes a mesh to allow airto pass therethrough.

[0021]FIG. 2 shows more the IOT of FIG. 1 including the output cavity 15from which an amplified high frequency signal is coupled. The capacitor1 gives a continuous distributed capacitance and has inherently lowinductance. It short circuits stray and leakage rf power that mayotherwise detrimentally modify the signal to be amplified.

I claim:
 1. A capacitor comprising: an electrically conductive tubehaving an outer surface; a layer of insulating material on said outersurface of said tube; and electrically conductive material on theinsulating material.
 2. A capacitor as claimed in claim 1 wherein saidtube provides support for said layer and said conductive material.
 3. Acapacitor as claimed in claim 1 wherein said insulating material has agreater length in the longitudinal axial direction of said tube thansaid electrically conductive material.
 4. A capacitor as claimed inclaim 1 wherein said insulating material covers substantially the wholeof the outer surface of said tube.
 5. A capacitor as claimed in claim 1wherein said layer of insulating material comprises a plurality ofsheets of said material.
 6. A capacitor as claimed in claim 1 whereinsaid insulating material is Kapton (Trade Name).
 7. A capacitor asclaimed in claim 1 wherein said conductive material is metal foil.
 8. Acapacitor as claimed in claim 1 wherein the length of said tube is atleast twice its diameter.
 9. A capacitor as claimed in claim 1 whereinsaid conductive material extends in an axial direction and is positionedcentrally with respect to the ends of said tube.
 10. A capacitor asclaimed in claim 1 and including an electrical connector connected tosaid conductive material at substantially the mid-point of the axialextent of said conductive material.
 11. A capacitor as claimed in claim10 wherein said connector is substantially annular and makes aconnection with the conductive material around the inner periphery ofsaid connector.
 12. A capacitor as claimed in claim 11 wherein saidconnector is an annular member extensive in a plane normal to thelongitudinal axis.
 13. A capacitor as claimed in claim 11 wherein saidconnector is an apertured annular plate.
 14. A capacitor as claimed inclaim 11 wherein said connector includes a mesh.
 15. A capacitor asclaimed in claim 10 wherein said connector is electrically connected tosaid conductive material via a plurality of fingers.
 16. A capacitor asclaimed in claim 10 wherein said connector is electrically connected toa wall surrounding said connector via a plurality of fingers.
 17. Acapacitor as claimed in claim 10 and including an electricallyinsulating support on which said connector is mounted.
 18. A capacitoras claimed in claim 1 and capable of holding off several tens ofkilovolts.
 19. A capacitor as claimed in claim 1 and comprisinganti-corona means at one or both ends of said tube.
 20. A capacitor asclaimed in claim 1 wherein said insulating material is dip coated onsaid tube.
 21. A capacitor as claimed in claim 20 wherein the insulatingmaterial is Kapton.
 22. An electron beam tube comprising an electron gunhaving a cathode and a capacitor comprising: an electrically conductivetube having an outer surface; a layer of insulating material on saidouter surface of said tube and electrically conductive material on theinsulating material, said capacitor being a bypass capacitor to shortcircuit stray and/or leakage high frequency power.
 23. An electron beamtube as claimed in claim 22 wherein said tube of said capacitor isconnected to cathode potential.
 24. An electron beam tube as claimed inclaim 22 and including an annular input resonant cavity arranged aboutthe electron gun, said capacitor being located within a volumesurrounded by an inner wall of said cavity and substantially parallelthereto.
 25. A tube as claimed in claim 24 wherein said conductivematerial of said capacitor is electrically connected to said inner wallof said cavity.
 26. A tube as claimed in any of claim 22 and being aninductive output tube (IOT).